Explosive atmosphere is generated, if flammable substances such as flammable gas (including vapor generated from flammable liquid) or flammable dust is mixed with air. If an electric device or an electronic device (hereinafter, simply referred to as a device) is used under such explosive atmosphere, explosive gas (flammable dust) may be burned by heat from a high-temperature part of the device or an electric spark generated in the device. Therefore, the device includes an explosion-proof structure so that explosive gas (flammable dust) is not burned even if it is used in the explosive atmosphere.
One example of such an explosion-proof structure of the device may be a pressurized explosion-proof structure. In such a pressurized explosion-proof structure, a main part of the device, which may be an ignition source, is arranged in a certain container (an explosion-proof container). Nonflammable gas (protective gas) such as nitrogen is supplied in the container to keep inner pressure and the container is filled with nonflammable gas. Thus, the ignition source is isolated from the atmosphere outside the container (explosive atmosphere). Nitrogen gas is generally used for the nonflammable gas supplied into the container, since nitrogen gas is easily handled and excellent in chemical stability.